This invention relates to a transfer device for use in an electrophotographic copying machine for transferring developing agent from the surface of a photosensitive body to an image receiving medium such as copy paper, and more particularly to such a transfer device having movable screening plates for preventing talc from becoming attached to the surface of the photosensitive body, thereby improving the quality of transferred images.
When an image is formed on a sheet of copy paper by an electrophotographic method, the developing agent attached on the surface of a photosensitive body by electrostatic force is transferred to the copy paper. In order to maintain the quality of transferred images high, therefore, care must be taken of the images on the photosensitive body such that foreign objects will be prevented from attaching on its surface. In general, an image receiving medium such as copy paper contains talc, for example, of magnesium silicate and talc particles tend to drop off mechanically from the edges when the paper is moved, becoming attached to the surface of the photosensitive body when there is a discharge in the transfer device. During a charging process, talc particles thus attached on the surface of the photosensitive body become charged with the polarity opposite to that of the developing agent and hence attract the particles of developing agent. As a result, unwanted images are formed on the copy paper corresponding to talc particles on the surface of the photosensitive body. Talc particles are generally smaller than the particles of the developing agent and cannot be removed and collected by the cleaner blade which is provided for cleaning the surface of the photosensitive body. This means that a polishing agent must be used separately to get rid of the talc particles attached on the surface and hence that extra efforts must be taken for the maintenance.